Helically coiled wire inserts are often used when fasteners are being fastened into relatively soft parent materials. For example, a wire insert may be introduced into a tapped hole in a relatively soft parent material, such as aluminum, to substantially reduce the risk of stripping the hole when a relatively hard fastener, such as a steel bolt, is received therein. Wire inserts are generally formed from a single length of wire that is wound into a helical shape, thereby defining a cylindrical channel including an internal and an external thread pattern. One end of the wire insert may include a tang, generally formed by bending one end of the length of wire substantially transversely across the cylindrical channel.
To install wire inserts, a prewinder tool may be used onto which a wire insert may be received prior to insertion into a tapped hole. For example, FIGS. 1-2C show a prewinder mandrel 10 for a prewinder tool (not shown) that includes a threaded lead end 12 terminating in a lead tip 14. A slot 16 is provided across the lead tip 14 for receiving a tang from a wire insert (not shown) therein. The slot 16 divides the lead tip 14 into a first end portion 22 having a helical bevel 24 defined by the thread pattern 20, and a second end portion 26 having an inclined ramp 28 and a leading edge 32. The prewinder tool includes a threaded nozzle (not shown) through which the mandrel 10 may extend, and the nozzle and mandrel 10 may include cooperating thread patterns for driving the mandrel 10 at a predetermined pitch.
To wind a wire insert onto the mandrel 10, the mandrel 10 is rotated about its longitudinal axis with respect to the wire insert, and the lead tip 14 is directed into the open end of the wire insert, through the cylindrical channel and towards the tang. The thread pattern of the lead end 12 substantially engages the internal thread pattern of the wire insert, generally compressing the wire insert radially as it is advanced over the lead end 12. When the lead tip 14 passes through the cylindrical channel, the tang of the wire insert 10 (not shown) is engaged by the leading edge 32 of the lead tip 14 and enters the slot 16, thereby fixing the wire insert on the lead end 12.
The lead end 12 may then be introduced into a tapped hole (not shown), and the mandrel 10 rotated further to direct the wire insert into the tapped hole, the external thread pattern of the wire insert cooperating with a thread pattern of the tapped hole. Once the wire insert is fully received in the tapped hole, the rotation of the mandrel 10 may be reversed, the wire insert unwound from the lead end 12, and the lead end 12 withdrawn from the tapped hole, leaving the wire insert therein. As the mandrel 10 is being rotated to unwind the wire insert, the tang may slide along the inclined ramp 28 and out of the slot 16.
One of the problems often associated with conventional prewinder mandrels is improper seating of the tang within the slot as the wire insert is wound onto the lead end. During use, a force is generally applied tangentially between the mandrel and the wire insert, e.g., along their cooperating thread patterns, to wind the wire insert onto the lead end and to insert the wire insert into a tapped hole. The substantial loads transferred between the mandrel and the wire insert may create risks of damage to the nozzle of the tool, the mandrel, individual inserts, and/or the tapped hole unless precise tolerances are maintained.
In addition, because the leading edge is generally higher than the remaining portions of the lead end, it may result in the tang being picked up too early by the slot. This may cause the tang to bend outward, may distort the shape of the wire insert, may increase the diameter of the tang end and/or may even cause the tang to break, substantially increasing the risk of jamming or cross-threading in the nozzle and/or in the tapped hole.
Furthermore, the leading edge may result in single point contact between the tang and the lead tip. If the geometry of this contact is altered, for example, due to poor mandrel or tool manufacture, wear or damage to the mandrel or tool, variation in wire insert shape, variation in tapped hole geometry, and the like, the load transfer between the mandrel and the insert may be altered significantly, and problems similar to those described above may occur.
Accordingly, there is a need for improved prewinder mandrels and/or tools for installing helically coiled wire inserts.